If really rich at part throttle, as others have said, probably a power valve problem. Backfiring can damage the powervalves and good idea to put the holly powervalve backfire preventer kit in to prevent damaging another one (there are posts on the forum that show how to do this).
Fix that first then see if still causing to not run well at full throttle. you will need to determine whether it is running rich or lean at full throttle too rich, and you may need to put smaller main jets (available through most speed shops or online) or go the other way if too lean . What main jets to you currently have in it?